1. Field of the Invention
The present invention is directed to a method of heat treating 9Ni-4Co-0.30C class steel alloys. More particularly, the present invention is directed to a method of heat treating such alloys which uses shortened times for normalizing, austenitizing, and tempering steps, as well as a lowered tempering temperature when compared to existing heat treatments.
2. State of the Art
In the discussion of the state of the art that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that theses structures and/or methods constitute prior art. Applicant expressly reserves the right to demonstrate that such structures and/or methods do not qualify as prior art against the present invention.
Existing heat treatments for 9Ni-4Co-0.30C class steel alloys, such as AMS 6526, AMS 6524, ASTM A579, and MIL-H-6875, prescribe heat treatments which employ long xe2x80x9ctime at temperaturexe2x80x9d periods for normalizing, austenitizing, and tempering of the alloys. This is especially true when heat treating a part or body made from the above-mentioned alloys, at least a portion of which having a relatively large sectional thickness. Generally, the above-mentioned heat treatments require approximately 1 additional hour of hold time to be added for each inch in part sectional thickness. For parts having regions with different sectional thicknesses, the times for heat treatment must be calculated based upon the maximum sectional thickness possessed by any portion of the part being treated.
It has been determined that application of existing heat treatments to parts made from 9Ni-4Co-0.30C class steel alloys, at least a portion of which having a relatively large sectional thickness, will in many cases fail to give the necessary strength and toughness properties necessary for certain high performance applications.
Existing heat treatments for 9Ni-4Co-0.30C class steel alloys-generally include a normalizing step comprising treating the alloy at a temperature of approximately 1650xc2x0 F. for 1 hour per every 1 inch in sectional thickness, and austenitizing step comprising treating the alloy at a temperature of approximately 1550xc2x0 F. for a period of 1 hour per every inch of sectional thickness of the part or body formed from the alloy, and a tempering step which comprises treating a body or part formed from the alloy at a temperature of approximately 1000xc2x0 F. for 2 hours plus an additional 1 hour for every inch of sectional thickness of the part or body.
It is an object of the present invention to overcome these and other problems associated with existing heat treating methods for 9Ni-4Co-0.30C steel alloys. Generally speaking, the present invention comprises a method of heat treating the above-mentioned alloys which utilizes shorter treatment times for the normalizing, austenitizing and tempering steps, as well as a lowered tempering temperature, when compared to the treatment times and tempering temperature of existing recommended heat treatments for this particular class of alloys.
In one aspect, the present invention provides a method of treating a steel body having a sectional thickness comprising the steps of (i) normalizing the steel body at a temperature of approximately 1650xc2x0 F. over a period of approximately 1-4 hrs.; (ii) austenitizing the steel body at a temperature of approximately 1550xc2x0 F. over a period of 1-3 hrs.; (iii) quenching the steel body from the temperature of approximately 1550xc2x0 F.; (iv) cooling and holding the steel body to a temperature of minus 100xc2x0 F. for a period of at least approximately 2 hrs.; and (v) tempering the steel body at a temperature of approximately 975xc2x0 F. for a period of approximately 2-3 hrs., cooling the steel body to ambient, then tempering the steel body again at a temperature of approximately 975xc2x0 F. for a period of approximately 2-3 hrs.